1. Field of the Invention
The present invention relates to phosphosilicate glass ceramic, and, more specifically, to a phosphosilicate glass ceramic which comprises fluoroapatite crystals and leucite crystals, is chemically stable, is esthetically very attractive, and is suitable for the preparation of dental restorations and in particular for pressing on dental alloys.
2. Description of the Related Art
Phosphosilicate glass ceramics are known in the prior art. In some cases, they contain fluoroapatite and/or leucite as main or secondary crystal phase.
German Patent No. 4423793 describes a phosphosilicate glass ceramic which, in addition to a leucite main crystal phase, comprises at least one further crystal phase and one or more glass phases. The secondary crystal phase can comprise rod- or needle-shaped apatite crystals, such as fluoroapatite, which have a length greater than 2 μm. However, a high CaO content is necessary to precipitate needle-like fluoroapatite crystals. Furthermore, the high leucite content of the glass ceramic necessitates relatively high processing temperatures of up to 1200° C. to mould the glass ceramic by pressing into the desired dental framework, i.e. to achieve a viscous flow of the glass ceramic. Moreover, the high leucite content results in a high linear expansion coefficient in the range of about 15×10−6K−1 to about 20×10−6K−1. The glass ceramic is therefore not suitable for the coating of materials with low expansion coefficients such as specific metal alloys.
German Patent No. 19725555 describes a translucent glass ceramic, the main crystal phase of which consists of fluoroapatite crystals. In addition, depending on the composition of the starting glass used, further crystal phases can be formed, but not a leucite phase. The glass ceramic has a thermal expansion coefficient of 6.0×10−6K−1 to 12.0×10−6K−1. This limits the use of the glass ceramic, as it can be employed only to coat or veneer dental framework made of materials with a very low expansion coefficient, such as lithium disilicate glass ceramics or titanium. In addition, very high processing temperatures of up to 1200° C. are also necessary to mould the glass ceramic by pressing into the desired dental structure.